North American X-15-2, Air Force serial 56-6671, made the program's first powered flight on September 17, 1959. After a 1962 emergency-landing accident it was rebuilt as the longer X-15A-2 with external propellant tanks and heat-protection provisions. On its 53rd and final flight, October 3, 1967, William Knight reached Mach 6.70, the program's absolute speed record; severe heating damage led to the aircraft's retirement.